PROJECT SUMMARY Severe maternal morbidity (SMM) includes serious threats to maternal health and survival that occur at delivery or postpartum. SMM, which includes postpartum hemorrhage, hypertensive disorders, sepsis, and other serious conditions, can be life-threatening and have long-term effects on a woman?s health as well as adversely impact her infant?s health and well-being. Over the past 15 years, the prevalence of SMM has doubled in the U.S., currently affecting more than 65,000 women each year. Additionally, the prevalence of SMM is highest in women of minority races and ethnicities, particularly women who are non-Hispanic black or Hispanic. However, the most important risk factors for SMM are not established and it is unknown what is contributing to racial/ethnic disparities and increasing trends in SMM. Our goal is to improve the understanding of SMM and its risk factors to provide evidence that supports efforts to reduce the prevalence of SMM and racial/ethnic disparities in SMM. The proposed project will use linked vital statistics, patient discharge, and U.S. Census data for over 4 million births in California from 2007 to 2014. We will test trends in SMM prevalence and calculate the population attributable risks for three key potential risk factors?advanced maternal age, prepregnancy obesity, and cesarean delivery?and their combination across the years of study (Aim 1). Additionally, we will examine a wide range of potential risk factors to identify those that are most important using a machine-learning/data-adaptive approach (Aim 2). Both study aims will be done in the overall sample and separately in racial/ethnic groups. Understanding SMM trends and determining the strongest predictors of SMM are critical steps to launch forward research of this pressing and understudied public health problem. Many factors may present crucial opportunities to prevent the development severe maternal morbidity?saving the lives of mothers and helping ensure a healthy future for their children.